The invention relates to a hydraulic system with a hydraulic machine, in particular an axial piston or radial piston machine, in which the moving, in particular rotating, machine components do not churn in fluid. The invention relates to all types of hydraulic machine in which leakage fluid occurs, especially for the purpose of lubrication, and is to be returned to the working circuit.
If hydraulic motors or pumps are operated in a housing filled with fluid, churning losses occur, and these increase with increasing rotational speed. Furthermore, the oil in oil-filled housings is made to foam by the moving or rotating components, causing problems, inter alia, in the onward movement of this oil into storage reservoirs. Such onward movement of the fluid with which the housing is filled is conventionally brought about by pumps or by excess pressure or a vacuum in the housing, forcing or sucking the fluid into the fluid storage reservoir of the working machine. The forced onward movement of the oil in conventional hydraulic systems is necessary since line losses due to flow resistance in the lines leading to the storage reservoir and, usually, also differences in level between the hydraulic machine and the reservoir have to be overcome. The reservoirs of conventional hydraulic working machines are normally arranged above the hydraulic motors, which are usually mounted on the axle to be driven, the said reservoirs being arranged significantly above the axle to be driven, for example.
The forced introduction of the oil into the storage reservoir leads to further turbulent mixing of the oil with the oil contained therein and the air present there. There is therefore further foaming of the fluid and hence a further increase in volume due to the inclusion of air in the oil. However, the inclusion of air in the fluid reduces the compressibility of the fluid, making it necessary to degas the fluid before the fluid removed from the housing can be returned to the working circuit. This is generally accomplished by allowing the fluid to settle in a storage reservoir of large dimensions in which the included air can escape from the oil/air mixture through the settling of the oil. For this purpose, large storage reservoirs are required since a certain time is required for the air to escape and fluid has to be removed continuously from the storage reservoir and fed to the working circuit, based on pressure or vacuum in the hydraulic machine. At the same time, there is a need to ensure that the fluid is removed without air, something that is not always the case in conventional machines.
The oil in housings of hydraulic machines, which is generally also referred to as leakage oil or leakage fluid, is formed essentially through leakage due to the lubrication and cooling of components that are moved relative to one another, especially the relative motion between pistons and cylinders, but also in the lubrication of sliding contact bearings. However, leakage can also occur in the control circuit for the hydraulic machine, which is likewise normally supplied with fluid by means of the charge pump. To enable all the moving components to be supplied with oil, the housing of the hydraulic machine is generally filled completely with oil, resulting in the churning losses already mentioned above. In conventional hydraulic machines, there is furthermore a need for pumping power to remove/circulate the oil in the housing, and this power is no longer available to the hydraulic system for doing work.
To reduce churning losses, it is advantageous, especially when operating at high rotational speeds, to remove the fluid from the machine housing so that the drive unit rotates in an “empty” housing, i.e. the rotating and moving machine components do not churn or run in oil. Hydraulic machines of this kind are also referred to as dry-case machines.
Various methods have been proposed for removing the fluid from the housing, all of them based on sucking or forcing the leakage fluid out by means of excess pressure. Here, use is generally made of pumps which either suck the housing of the hydraulic machine dry or produce an excess pressure in the housing, thereby sucking or forcing the leakage oil out of the housing of the hydraulic machine into a storage reservoir. In order to ensure that the housing is sucked or pumped dry, the additional pumps used for this purpose are overdimensioned. As a result, too much oil is generally sucked out of the housing of the hydraulic machine, as a result of which an unnecessary amount of air is taken along and fed to the reservoir. The effect of this air, especially due to the intended complete removal of continuously inflowing leakage oil, is increased foaming of the oil in the reservoir and, in many cases, spilling over of foam in the reservoir.
DE 41 28 615 C1, for example, has proposed arranging a pumping device between the case drain port and the reservoir for the purpose of sucking leakage oil out of a housing in order to ensure that the drive unit components do not run in leakage oil. Here, the leakage oil is withdrawn completely and continuously, together with air, out of the bottom of the housing of the hydraulic machine and forced to a reservoir by means of an additional leakage oil pump.
DE 42 15 869 C1 discloses the arrangement, in the interior of the housing, of a pumping device which is in drive connection with the drive unit and is provided for the purpose of pumping leakage oil out of the housing and into a reservoir.
DE 44 14 509 C1 has furthermore proposed a method for removing fluid from a housing of a hydrostatic machine, in which the foamed leakage fluid is removed from the housing by supplying excess pressure or applying a vacuum to the housing and is forced to a reservoir. The power required for the excess pressure and vacuum pump must be provided by the main drive engine, e.g. an internal combustion engine, and is thus no longer available for the hydraulic drive.
To enable the leakage fluid or working fluid to be sucked out of the housing by means of any pump device, the said device must be designed for the full leakage fluid flow. The required power for the suction pump must therefore be correspondingly large and, as a result, the desired power saving through the avoidance of churning losses is significantly lower than the churning losses saved. Furthermore, the additional pump requires a considerable installation space, either within or outside the drive unit.
The leakage oil removed from the hydraulic machine is then passed into a reservoir, in which the hydraulic fluid can settle and release included gas particles. Since this takes a certain time, the swirled volume of fluid is not available to the working circuit, the result being that the quantity of hydraulic fluid is greater than is actually needed for the working circuit to be supplied by the charge pump.
The more time the fluid forced into the reservoir has to allow the gas included in it to escape, the better is the compression behaviour of the fluid and hence the efficiency of the hydraulic drive. Thus the oil reservoirs in conventional hydraulic systems are of large dimensions to ensure that the air has sufficient time to escape from the oil and hence that as little gas/air as possible is pumped into the working circuit by the charge pump that sucks the working fluid out of the storage reservoir.
In all the systems previously presented, all of the working fluid that occurs in the housing as a result of leakage is forcibly removed from the housing of the hydraulic machine and fed to a reservoir. In these systems, the leakage fluid is already carrying gas particles as it emerges from the housing, and these are foamed even more, together with the oil, by the rotation of the rotating machine components or, at the latest, when they are introduced into the reservoir. In the reservoir, there is the further effect that the reservoirs become clogged with foam over time. Foam is formed that does not break down again, even after a long settling time. Over time, therefore, the reservoir becomes clogged with foam. This additional volume required by such foam must also be taken into account when designing the reservoir volume.
It is the object of the invention to provide a hydraulic system in which at least the rotating machine components do not churn in oil, and an additional pump for removing leakage oil from the hydraulic machine and foaming of the leakage oil are avoided. At the same time, mixing of gas or air with the leakage fluid shall be avoided.
The object is achieved with a hydraulic system according to Claim 1. The subclaims that depend on Claim 1 are directed to advantageous embodiments of the hydraulic system.
The object is likewise achieved with a hydraulic drive according to Claim 9. The subclaims that depend on Claim 9 are directed to preferred embodiments of the hydraulic drive.